1. Field of the Invention
The present invention relates to an apparatus for controlling the exhaust timing of a two-cycle engine and more specifically to an apparatus for changing the exhaust timing of the exhaust port by use of a control valve.
2. Prior Art
It is well known that the exhaust timing of two-cycle engines is always constant with respect to the crank angle because the exhaust port is opened and closed by the reciprocating motion of the piston itself. Therefore, the engine, if its exhaust port timing is tuned such that a large power is obtained at high engine speeds, has a tendency to cause so-called "short-circuiting" phenomena in which part of mixture gas straightly goes through the cylinder and is discharged from the exhaust port at low engine speeds due to a relatively elongated opening and closing timing of the exhaust port. On the contrary, in case where the exhaust port timing is adjusted such that a smooth driveability is obtained at low engine speeds, the engine power goes down at high speed ranges due to the decreased exhaust efficiency.
To solve this problem with two-cycle engines, miscellaneous techniques have been proposed in which the exhaust timing is positively controlled so as to obtain a stable power over a wide range of the engine speed. In Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-177502, the applicant of the present invention discloses a technique in which a control valve moves slidably up and down along the exhaust port and more in detail, a control surface formed at the tip of the control valve moves up and down along the exhaust port with a minimum gap retained so that the short-circuiting from the edges of the exhaust port and the control surface is prevented.
The exhaust timing control is not an adequate means to raise the engine power. For example, there is a known technology in which a resonance chamber is provided at the exhaust port in addition to the control valve. In this technology, the resonance chamber is constituted to open at low engine speeds to enhance the engine power by the exhaust resonance effect and to close at high engine speeds to prevent the power reduction.
However, the problem of this technology is that the exhaust timing control and the opening and closing control of the resonance chamber must be performed separately from each other, this resulting in making the control complicated and increasing the number of components.
Further, with respect to the exhaust timing control apparatus, there is a technique in which the control valve has a forward and backward motion by a diaphragm secured to a shaft of the control valve. The diaphragm has a pressure chamber on the opposite side of the control valve, into which an exhaust pressure is introduced. When the engine is operated at low speeds, since the exhaust pressure is relatively low, the control valve is moved forward so as to restrict the exhaust port, i.e., to lower the upper edge of the exhaust port and when the engine is operated at high speeds, since the exhaust pressure is relatively high, the control valve is moved backward so as to open the exhaust port, i.e., to raise the upper edge of the exhaust port.
This type of the exhaust timing control apparatus has a problem that the exhaust pressure is generally low and therefore enough pressure to operate the control valve can not be obtained particularly at low and medium speeds of the engine.